Protective system



Feb. 15, 1944. MacCARTHY PROTECTIVE SYSTEM Filed Jan. 2, 1941 A Mm -|& 55 F .00....

Inv ehto r Donne! l' D. FIacCaTt I192 b g .37 2%: H iZ /Z-ILQP negPatented Feb. 15, 1944 PROTECTIVE SYSTEM Donnell D. MacCarthy.Pittsfield, Mass assignor to General Electric Company, a corporation ofNew York Application January 2, 1941, Serial No. 872,780

6 Claims. (Cl. 175-294) gized from a suitable source, such as through aconstant current transformer. In order to prevent all the lamps whichare connected in series from failing to light when one of the lamps inthe circuit is burned out, there are usually provided suitable devices,such as film cutouts which are connected in shunt with each of thelamps. These film cutouts are designed to breakdown at a voltageslightly higher than the rated voltage of the lamp so that when a lampburns out its cutout will provide a shunting circuit for the damagedlamp. It has been found, however, that in operation film cutouts arebroken down or short circuited by impulse voltages that appear on thefeeder due to lightning. Thus, lamps are put out of service althoughthey are not burned out. It has, therefore, been the practice after eachlightning storm to check the film cutouts and replace those which haveburned out.

It is, therefore, an object of my invention to provide an improvedsystem for protecting a low voltage device from impulse failure.

Another object of my invention is to provide an improved system forprotecting film cutouts which may be employed to shunt seriallyconnected street lamps from becoming short circuited when a high voltagewave passes over the line.

A further object of my invention is to provide an improved protectivedevice for street lamps which is simple in construction, reliable inoperation, and which is easy to install.

Further objects and advantages of my invention will become apparent fromth following description referring to the accompanying drawing, and thefeatures of novelty which characterize my invention will be pointed outwith particu larity in the claims annexed to and forming a part of thisspecification.

In the drawing, Fig. 1 illustrates a plurality of serially connectedlamps, such as may be found in a street lightning circuit, each of thelamps having connected therewith a protective system which is providedwith an embodiment of my invention, and Fig. 2 illustrates theprotective system employed in Fig. 1.

In the arrangement illustrated in the drawing, I have provided each lampwith one of my protective devices for preventing high voltage waves frombeing impressed across and damaging the lamp or the film cutout whichwould put the lamp out of service. This protective device or systemincludes a coil connected in series with one 'of the lines feeding thelamp, and a spark gap and a capacitor connected across the lines, thecapacitor being connected across the feeder lines between the coil andthe low voltage device or cutout and the gap being connected across thefeeder lines on the other side of the coil.

Referring more particularly to Fig. l of the drawing, I have illustrateda circuit including a plurality of lamps III which are connected inseries, such as may be found in conventional street lighting circuits.The lamps are connected in series with the feeder lines II and i2, theselines being in turn connected to any suitable source of power. Thus,these lines are connected to a secondary winding I 3 of a constantcurrent transformer I which has a primary winding 15 connected to asuitable source of potential. A conventional lightning arresterindicated generally by the numeral "5 may have one side thereofconnected to any suitable point in the series system, such as at H,through a conductor IS, the other side of this arrester being grounded.Since the lamps it are connected in series, a suitable low voltagedevice is connected across the lamp terminals, and may be contained inthe lamp sockets, so that if one of the lamps should burn out the lowvoltage device will are over and pro- .vide a shunting circuittherearound in order that the remaining lights may be supplied withpower. Such a low voltage device may be a conventional film cutout whichis indicated by the numeral IS, in Fig. 2. Since street lamps areusually designed to operate at relatively low voltages, any device whichis to provide a shunting path for the lamp in case it burns out mustalso be a low voltage device. In order, therefore, to accomplish thisshunting purpose in case the lamp burns out, the film cutout must bedesigned to breakdown at a voltage not greatly in excess of the normallamp operating voltage, and it must not breakdown at voltages equal toor less than the operating voltage of the lamp. Thus, film cutouts havebeen designed to breakdown at voltage ranges, such as from 50 to volts,from to volts, and from 250 to 300 volts, depending upon the rating ofthe lamp which it shunts. It will be seen, therefore, that a highvoltage wave traveling over the feeder lines will tend to cause impulsefailure of the film cutouts. Such high voltage waves are usually causedwhen lightning strikes the feeder lines in some place or result frominduction due to a stroke near the lines. However switching may causetravel- Y ing waves that will damage the lamps or film film cutout. Inorder, therefore, to provide an arrangement for protecting a low voltagedevice, such as a film cutout from impulse failure. I have provided asystem which, as shown in Fig. 2, includes an inductance coil 20, acapacitor 2|, and a suitable spark gap arrangement 22. This protectivesystem or device is placed electrically relatively close or adjacent tothe film cutout in order that any traveling wave will have to passthrough the protective system before it can be impressed on the filmcutout l9. In order that the protective device may be easily installedacross each of the serially connected lamps it is placed in a suitablecasing indicated generally by the letter A to provide a unitarystructure and has line leads 23 and 24 and load leads 25 and 26extending therefrom. The load leads, therefore, may be connected acrossthe terminals of the lamp film cutout combination while the line leads23 and 24 may be connected to the feeder lines. Thus, as illustrated inFig. 1, the line lead 23 is connected to-the incoming line H while theother line lead 24 is connected to a cable running to the next seriallyconnected lamp. It will-be seen, therefore, that when'a protectivedevice is connected across a film cutout lamp combination that theinductance 20 is connected in series with or between line lead 24and-load lead 26 or in series with one of the lines, and the capacitor21 is connected across the lines and between the inductance and the filmcutout, while the spark gap 22 is connected across the lines on the sideof the inductance remote from the lamp and film cutout to be protected.

In Fig. 1 let us assume that a traveling wave originates at any pointsuch as X on the feeder. This impulse will divide and part will go tothe right and pass through the lamps or their shunt protective devices,until it is dissipated through the ground connection of the streetlighting circuit somewhere to the right of the point X. The other partof the wave will travel to the left of X passing through the lamps orthe protective devices that shunt them. One or more conventionallightning arresters may be connected between the-feeder line and groundas shown at point I! in Fig. 1. Such arresters can never in whollyeffective indissipating the entire traveling wave to ground since anovervoltage is required to start the discharge in the arrester andbecause an overvoltage will exist when the arrester is discharging dueto the impedance of the arrester and the impedance of the groundconnection. This part of the surge will pass by such arresters andcontinue along the line, and will pass through the series lamps or theirparallel protective devices. In the absence of a protective device, suchas has been described, the part of the surge passing by the arresterswould be capable of damaging the lamps or film cutouts. It is to benoted thatconventional arrestof 2 kv. per microsecond or slower.

ers, such as indicated by the numeral It in Fig.

1, are therefore incapable of protecting the street lights or filmcutouts from impulse failure, even though a large number of thesearresters were to be used. For this reason such arresters are usuallynot used on-the street lighting circuits at a distance from the powersource. However, such arresters, if properly applied to the ungroundedterminal of the secondary winding of the constant current transformerl3, can protect this winding from damage even though these arresters areineffective in protecting the street lights or film cutouts from damage.However, my protective device will be effective in preventing damage tothe lamps or film cutouts, irrespective of whether or not conventionallightning arresters are connected between the feeder and ground. In theapplication of my inventionto a protective system such as for lampswhich are seri ally connected as in street lighting (circuits, the sparkgap may be of any suitable type. set to a small spacing to enable it tolimit the voltage to as low a value as is possible. Mica spaced gapshave been found effective for this purpose due to the low impulse ratio.However, the spacing cannot be so small as to permit the gap to becomeshort circuited by the droplets of metal melting from the electrode bydischarges that the gap must carry in service. Thus, the gap will be setto breakdown consistentlyat approximately the same voltage or at about2.3 kv., 60 cycles, or at about.3.8 kv., crest voltage at rates Themaximum voltage which may be impressed across the cutout I9 is, ofcourse, determined by the film cutout itself which is a relatively lowvalue and the proper values of inductance 20 and capacitance 2! with agap 22 set for as small a spacing as possible, therefore, may bedetermined by the following equations developed below:

The first part of the front of most impulse voltage waves can beapproximated by assuming that the voltage rises uniformly. Such anassumption will be sufiiciently accurate over the range of voltage from0 to about 4000 volts where the gap in this protective device will sparkover. In calculating the necessary values of L and C for the protectivedevice, it is also permissible to neglect the resistance capacity, andinductance of the film cutout and the lamp since ignoring these factorswill result in a conservative design. The electrical circuit with whichwe are dealing may have the following symbols:

E=the impulse voltage at time t E1=rate of rise of voltage which isassumed to be constant Ee=voltage across capacitor, lamp, and filmcutout at time t t=time in s L=inductance in h C=capacity in pf m t amat a (cot which is an approximation consisting oi'the first two terms orthe infinite series expression for a sine function.

a-(a% (a) By combining Equations 1 and 3, Equation 4 is obtgined ind (aIn calculating a suitable protection by the use of Equation 4, let usassume that the gap operates at an impulse voltage of E=4000 v. in thetime t and that the voltage applied to the lamp and cutout, Es, shouldnot exceed 100 v. Then it is possible to use Equation 4 to obtain thenecessary values of the product of L and C which are included in thetable below for tour diiiercut rates of voltage rise.

Rate oi rise Time required Necessary value oi impulse ior voltage E toof LXO to limit voltage reach 4.000 V. E. to 100 v.

1 Where L is in microhenrles and C is in microisrads.

Thus it will be seen that in order to hold the voltage on the lamp andcutout to 100 v., the

product of L and C must be made larger it protection is desired at slowrates of voltage rise, such as 1 ,kv./]J.8 than for fast rates of risesuch as 1000 kv./ .is. Available information on lightning transientsindicates that only a small percentage of impulse voltages on feederswill have a slower rate of voltage rise than 1 kv./ns. Hence if aprotective device were designed to have a LC product of the order of 100or more, the voltage appearing on the lamp and cutout very rarely willexceed 100 v., or only in case the rate of voltage rise is slower than 1kin/ s. The proper value of the LC product in a practical arrester is,of course, determined by a consideration of economics, since it isnecessary to balance the value of protection obtained for very slowwaves which is obtained by using larger values of L and C against thehigher cost and diminishing returns which will be obtained.

It will be seen from Equation 4 that if a small value of capacity wereused, a larger value of L will be necessary'in order to obtain thedesired LC product. However, it will not be possible to omit thecapacitor entirely since any practical form of inductance has a smallbut real value of terminal-to-terminai capacity. However, it thecapacitor II were omitted, the charge passing through theterminal-to-terminal capacity oi the inductance would result in avoltage across the film cutout suflicient to cause failure. However, theterminal-to-terminal capacity of the inductance L will not be a causeoifailure provided a relatively large capacitor is connected in parallelwith the lamp and cutout. However, in selecting the proper value of thecapacitor, above the needed minimum value, to be connected in parallelwith the lamp and cutout, the relative costs of capacity and inductancemust be balanced and it will be found uneconomical to make L too largeand C too small.

Modifications oi the particular arrangement which I have disclosedembodying my invention will occur to those skilled in the art, so that Ido not desire my invention to be limited to the par- 78 ticulararrangement set forth and I intend in the appended claims to cover allmodifications which do not depart from the spirit and scope oi myinvention.

What I claim as new and desire to secure by Letters Patent 0! the UnitedStates is:

l. A protective system including line leads adapted to be connected tofeeder lines, load leads adapted to be connected to a relatively lowvoltage device, a coil, a spark gap, and a capacitor, said coil onlybeing connected in series with one of said line leads and one of saidload leads, said capacitor being connected across said load leads on theload side of said coil, and only said gap being connected across saidline leads on the line side of said coil in order to protect the devicefrom impulse failure.

2. A system for protecting a relatively low voltage device fromlightning surges including line leads adapted to be connected to feederlines, load leads adapted to be connected to the device, a coil, a sparkgap, and a capacitor, said coil only being connected in series with oneor said line leads and one of said load leads, said capacitor beingconnected across said load leads on the load side or said coil, and onlysaid gaD being connected across said line leads on the line side of saidcell in order to protect the device from impulse railure.

3. A protective system for a device the normal operating voltage ofwhich being within a, range oi approximately 50 to 300 volts includingline leads adapted to be connected to feeder lines, load leads adaptedto be connected to the device, a call, a spark gap. and a capacitor,said coil only being connected in series with one of said line leads andone 01 said load leads, said capacitor being connected across said loadleads on the load side of said coil, and only said gap being connectedacross said line leads of the line side .of the coil in order to protectthe device from impulse iailure. g 4. A protective system for a devicethe normal operating voltage of which being within a range ofapproximately 50 to 300 volm including line leads adapted to beconnected to feeder lines. load leads adapted to be connected to thedevice, a coil, a spark gap, and a capacitor, said coil being connectedin series with one of said line leads and one or said load leads, saidcapacitor being connected across said load leads on the load side ofsaid coil, and said gap being connected across said line leads on theline side of said coil in order to protect the device from impulsefailure, the approximate value oi the product oi the inductance of thecoil and the capacitance of said capacitor being determined by thefollowing equation:

i E 6L0 where Es equals the safe voltage which may be impressed acrossthe device, E equals the impulse spark potential or said gap due to avoltage traveling along the lines assuming it has a constant rate 01rise, t equals the time at which the voltage of the impulse reaches thevalue E, and L and C equal the values of the inductance and capacitance,respectively.

5. In a protective system for a street lighting circuit having a lampand a film cutout, line leads adapted to be connected to feeder lines,load leads adapted to be connected to the lamp, a coil, a spark gap, anda capacitor, said coil only being connected in series with one of saidline leads and one of said load leads, said capacitor being connectedacross said load leads on the load side of said coil, and only said gapbeing connected across said line leads on the line side or said coil inorder to protect the film cutout from tailures due to lightning surges.

8. A rotectivedevice for a film cutout having feeder lines connectedthereto including a coil, a spark gap, a capacitor, device to provide aunitary structure, a pair or line leads and a pair of load leadsextending from said casing, said coil being connected between one ofsaid line leads and one or said load leads,

a casing enclosing saidv said capacitor being-connected across saidleads on'the load side oi sald coil, said spark gap being connectedacross said leads on the line side of said coil so as toprotect saidcutout from impulse failure when said load leads of said protectivedevice are connected to the cutout and said line leads are connected tothe lines feeding the cutout, the product of the inductance of said cellin microhenries and the capacitance of said gsgacitor in microfa'radsbeing of the order of DONNELL D. MAOCAR'I'HY.

